This invention relates in general to bandgap reference circuits and, more specifically, to devices and methods for providing bandgap reference circuits with low temperature coefficients.
As shown in FIG. 1, a conventional bandgap reference circuit 10 includes a pre-regulator 12 that generates a regulated voltage VREG off the supply voltage VCC using a pair of current-mirror transistors Q1 and Q2, a resistor R1, and a set of series-connected diodes D1, D2, and D3. In addition, a start-up circuit 14xe2x80x94consisting of a bias transistor Q3, another set of series-connected diodes D4 and D5, and a resistor R2xe2x80x94biases a pair of VBE-differential transistors Q4 and Q5 at start-up, after which the transistor Q3 shuts off, thereby effectively isolating the start-up circuit 14 from the rest of the bandgap reference circuit 10.
Together, a current source transistor Q9 and a VBE-differential circuit 16 generate a differential voltage VDIF having a positive temperature coefficient from the regulated voltage VREG using a pair of current-mirror transistors Q6 and Q7, the VBE-differential transistors Q4 and Q5, a pair of resistors R3 and R4, and a driver transistor Q8. As a result, the bandgap voltage VBG output from the bandgap reference circuit 10 across a resistor R5 equals the differential voltage VDIF plus the base-emitter voltage VBE of the transistor Q5. Because the base-emitter voltage VBE has a negative temperature coefficient, any variations in the base-emitter voltage VBE due to temperature are countered by variations in the differential voltage VDIF, so that the bandgap voltage VBG should be relatively temperature independent. Unfortunately, the negative temperature dependence of the diodes D1, D2, and D3 makes the regulated voltage VREG relatively temperature dependent, which, in turn, makes the bandgap voltage VBG relatively temperature dependent.
Accordingly, there is a need in the art for an improved bandgap reference circuit that has a low temperature coefficient.
In accordance with this invention, a pre-regulator for generating a regulated voltage for use in generating a bandgap voltage from a bandgap reference circuit includes a current source (e.g., a wilson current source) and a VBE multiplier that receives current therefrom and generates/clamps the regulated voltage. Also, feedback circuitry regulates the current flow from the current source in response to feedback from the bandgap voltage.
In other embodiments of this invention, the pre-regulator described above is incorporated into a bandgap reference circuit.
In still another embodiment of this invention, a reference voltage is generated by driving a current into a VBE multiplier to generate and clamp a regulated voltage. The current is regulated in response to feedback from the reference voltage. Also, a VBE differential voltage is generated from the regulated voltage using a VBE differential circuit, and the reference voltage is generated from the VBE differential voltage and a base-emitter voltage drop.